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There are three principal ways that a sign can denote its objects. These are usually described as kinds, species, or types of signs, but it needs to be understood that the same thing can be a sign in different ways.

Beginning very roughly, the three main ways of being a sign can be described as follows:

An icon is a sign that denotes its objects by virtue of a quality that it shares with its objects.

An index is a sign that denotes its objects by virtue of an existential connection that it has with its objects.

A symbol is a sign that denotes its objects solely by virtue of the fact that it is interpreted to do so.